Optical data discs are a popular media choice for the distribution, storage and accessing of large volumes of data. This includes audio and video program material, as well as computer programs and data. Formats of optical data discs include audio CD (compact disc), CD-R (CD-readable), CD-ROM (CD-read only memory), DVD (digital versatile disc or digital video disc) media, DVD-RAM (random access memory), various types of rewritable media, such as magneto-optical (MO) discs (including near field recording technology), and phase change optical discs. In general, optical discs (such as CD-ROMs) are produced by making a master which has physical features representing the data formed in or on a reference surface therein. The master is used to make a stamper, which, in turn, is used to make production quantities of replica discs, each containing the data and tracking information which was formed in the master. The high data capacity, convenience, and relatively low production costs of such discs have contributed to their great success and acceptance in the marketplace.
In optical discs, data is stored as a series of lower reflectance "pits" embossed within a plane of higher reflectance "lands". The microscopic pits are formed on the surface of the plastic disc when the disc material is injected into a mold. Typically, the pitted side of the disc is then coated with a reflectance layer, such as a thin layer of aluminum, and in the case of a CD, followed by a protective layer of lacquer. The pits on an optical disc can be arranged in a spiral track originating at the disc center hub and ending at the disc outer rim, wherein each data track is defined as a cycle of the spiral. The data can also lie in a series of concentric data tracks spaced radially from the center hub.
To read the data on an optical disc, an optical disc player shines a small spot of laser light through the disc substrate onto the data layer as the disc rotates. The intensity of the light reflected from the disc's surface varies according to the presence (or absence) of pits along the information track. When a pit lies directly underneath the "readout" spot, much less light is reflected from the disc than when the spot is over a flat part of the track. A photodetector and other electronics inside the player translate this variation into the 0s and 1s of the digital code representing the stored information.
As optical disc technology has evolved, optical discs have increased in storage capacity. Higher density discs have resulted in the storage of a greater amount of information within the same size of disc area. For example, a CD having a storage capacity of 0.65 gigabytes has data pits which are 0.83 .mu.m long and has a track pitch (the distance between data tracks) of approximately 1.6 .mu.m. In comparison, a DVD disc data pit is as small as 0.4 .mu.m long, and a track pitch of only 0.74 .mu.m, resulting in a storage capacity of 5 gigabytes on a single layer. Similarly, MO and phase change disc track pitch varies with the density or storage capacity of the disc.
To read high-capacity optical discs having smaller pits and a smaller track pitch, the optical disc player's read beam must achieve a smaller spot focus. Further, data must be more precisely located on the optical disc substrate. Ideally, the data tracks are concentrically located about the center hole of the disc. During the optical disc manufacturing process, a centering error is introduced into the radial positioning of the data tracks (or track cycles) on the optical disc. This error is known as radial total indicated runout (RTIR). RTIR is defined as the measure of non-concentricity of the data tracks to the drive spindle on the optical disc player.
In a conventional optical disc manufacturing process, RTIR error is introduced during the injection molding process. A typical injection molding process begins with a tooling mechanism. The optical tooling mechanism includes a fixed side and a moving side. The moving side typically includes a stamper for replicating data and format information into the disc substrate, and a movable gate cut for cutting a central opening in the disc substrate. The stamper is located by an inner holder, wherein the inner holder fits over the stamper. Several more parts may be located at the center inside diameters of the tool. In typical optical tooling, all of these parts need to remain concentric between the gate cut and the removable inner holder for concentric registration (or centering) of the format information (data tracks) in the disc substrate relative to the disc central opening or center hole.
In a disc molding process, a resin, typically polycarbonate, is forced in through a sprue channel into a substrate cavity within the optical tooling (mold) to form the optical disc substrate. The format of the grooves and pits are replicated in the substrate by the stamper as the cavity is filled. After filling, the gate cut is brought forward to cut a center hole in the optical disc. After the part has sufficiently cooled, the optical tooling mold is opened and the sprue and product eject are brought forward for ejecting the formed optical disc off of the stamper. The inner holder may be removed to allow change out of the stamper.
Any misalignment of the aforementioned optical tooling results in the replication of greater RTIR error in the molded disc. Further, any debris, flash or other imperfections resulting from the gate cut action, and any misalignment of the moving stamper relative to the fixed side of the optical tooling will add to the RTIR error. When track pitch is larger, such as in CD optical discs, the disc reader is capable of reading optical discs having typical RTIR errors between 50 and 100 .mu.m due to a relatively large track pitch (1.6 .mu.m). For higher capacity discs, such as DVD discs, it is difficult (or impossible) for an optical reader to read a DVD optical disc having an RTIR error greater than 50 .mu.m, due to the smaller track pitch. Similar problems exist with MO disc technology having a typical RTIR between 20 and 30 .mu.m
In order to reduce the RTIR error to acceptable (or readable) levels, hubs are installed within the center opening of the optical disc. A new center is located, and the hub is installed centered on the disc relative to the formatted data tracks. This is typically accomplished using a costly centering process. Further, the hub itself is insert molded, resulting in a high expense relative to the total disc cost.
It is desirable to have an optical storage system including a high-capacity optical disc having a low RTIR error. It is desirable to have a high-capacity optical disc having low RTIR which utilizes an economically efficient method of centering the drive to the information on the disc. Further, it is desirable to have a disc molding process for forming high-capacity optical discs which may include simple modifications to conventional optical tooling, and which introduces low RTIR error into the disc substrate.